Such a phase plate is known from “Phase Contrast Enhancement with Phase Plates in Biological Electron Microscopy”, K. Nagayama et al., Microscopy Today, Vol. 18 No. 4, July 2010, pages 10-13, further referred to as Nagayama [-1-], and is also known as a Zernike phase plate.
In a Transmission Electron Microscope (TEM) a sample is imaged by passing a beam of energetic electrons with a selectable energy of, for example, between 40 keV and 400 keV, through the sample. For so-called weak-phase samples, such as biological samples, most electrons pass through the sample while some electrons are elastically or inelastically scattered, the elastically scattered electrons forming diffracted beams. The image is formed by interference of the elastically scattered and unscattered electrons (diffracted and undiffracted beams).
A problem arises in that the Contrast Transfer Function (CTF) for low spatial frequencies in the image is zero or close to zero, resulting in low visibility of large objects/structures. This is caused by the fact that a camera or fluorescent screen at the image plane is sensitive to intensity variations, but not to phase variations of the impinging electron beam.